In the deposition of metal onto semiconductor wafers in semiconductor manufacturing, plasma sources are used, in addition to a primary sputtering plasma source, to improve the performance of the processing tool. For instance, in ionized physical vapor deposition (iPVD) of metal onto a wafer, a supplemental plasma source is often used to increase plasma density to produce an increased fractional ionization of the metal atoms being deposited. This, combined with RF bias on the substrate wafer leads to improved coverage of the features on the wafer surface, and reduces the closing-off of the necks of such features. Likewise during a soft-etch stage that accompanies many iPVD and other processes, an auxiliary plasma source is employed to provide a dense plasma at the wafer level that will maintain a uniform etch at an acceptable rate and at a DC bias voltage level that is low enough to eliminate plasma damage to the wafer. Further, during a pre-clean stage of various processes, it is again desirable to perform a soft-etch of the wafer. As in the case of the soft-etch process, plasma density and uniformity are desired, as is process throughput and reduced plasma damage to the wafer.
In iPVD, the additional plasma source includes an RF antenna, mounted either internally or externally to the process volume. Both antenna locations have advantages and disadvantages. Where a soft-etch process step is executed in an iPVD module, RF power is applied principally to a wafer that is being processed to generate a plasma and provide a DC biasing voltage to the wafer. This method produces a uniform plasma above the wafer, but at high biasing voltages, in excess of 100 V. In such a case, a small inductively coupled plasma (ICP) improves the etch rate and reduces the bias voltage. Often, such a plasma also dramatically reduces the etch uniformity. In pre-cleaning applications, the pre-cleaning process is usually performed in a separate process module. Typically, the source for the plasma for the pre-cleaning includes an external ICP coil, which transmits power into a vacuum chamber through a dielectric window. To prevent the sputtered metal from coating the window, which would reduce RF penetration into the chamber, the window is often protected by a deposition baffle or shield of some complexity.
RF plasma sources for iPVD can use an internal coil design where the coil is immersed in the plasma. Due to high voltages developed on such a coil, the plasma sputters the coil, necessitating coil replacement. To reduce coil voltages and coil sputtering, the coil is typically operated at about 450 kHz. In addition, due to the large plasma thermal load, the coil needs to be water-cooled, complicating coil construction.
An alternative approach to the internal coil in iPVD is the external antenna design, one version of which has been adopted by Drewery et al. in U.S. Pat. No. 6,287,435. In this approach, the RF coil is external to the processing chamber and is situated in air. An RF field from the antenna penetrates into the chamber through a dielectric window. To preserve the transparency of the dielectric window to the RF, the window may be shielded from the metal ions in the plasma in a manner that still provides for RF transparency. This shielding of the window is solved by the introduction of a deposition baffle or shield. The shield is designed to be opaque to the large majority of the metal ions. As discussed in Drewery et al., this is accomplished by introducing a shield of electrically conductive material with long slots generally perpendicular to the direction of the RF antenna conductor segments. This approach removes some of the problems of the internal coil design, but suffers from the drawbacks of deposition shield complexity. The deposition shield must accomplish two counteracting goals: RF transparency and metal opacity. These goals are not easily accomplished, as one is usually achieved at the expense of the other.
The prospect of reduced RF transparency of a dielectric window creates a need to protect the dielectric window from metal deposition. The protection may be provided by a deposition shield, which is either made of one piece with a complex slot shape (such as a chevron) or a two-piece assembly with overlapping slats. Both types of shields introduce transmission losses and result in reduced efficiency of the RF source. The reduced efficiency, and its effect on the output of the whole RF source, can be offset by an increase in coil current, which in turn increases the voltage across the coil. This in turn leads to increased complexity of other parts of the RF circuit, such as the tuning network and RF connectors from the coil to the tuning network.
At iPVD pressures in the range of 30-100 mTorr and higher, an intense plasma having a density in the order of 1012 cm−3 and higher, and electron temperature of 4-5 eV is produced near the RF source. As a result, the shield is subjected to a large thermal load: up to 20 percent or even higher of the RF power is deposited back to the shield in the form of a plasma thermal load. This is particularly the case in deposition applications, for example, in ionized physical vapor deposition (iPVD) applications. These problems are also applicable to soft-etch and pre-clean applications, although these applications usually involve lower power, which simplifies RF power handling and component cooling requirements.
Accordingly, there is a need for an RF power source that overcomes the drawbacks set forth above.